With a demand for higher density in magnetic recording in recent years, the perpendicular magnetic recording method by which a magnetic field is recorded in the thicknesswise direction of a magnetic recording medium has attracted a great deal of attention. The magnetic material used in the perpendicular magnetic recording method must have an easy axis of magnetization in a direction perpendicular to the surface of the recording medium.
Hexagonal ferrites having uniaxial magnetocrystalline anisotropy, for example, barium ferrite (BaFe.sub.12 O.sub.19) magnetic powder, are hexagonal plate crystals, and have an easy axis of magnetization in a direction perpendicular to the plate surface. The plate surface of such a magnetic powder hence tend to become parallel to the surface of a substrate by the mere coating of it. Since its easy axis of magnetization orients in a direction perpendicular to the substrate surface with ease by a magnetic orientation treatment or mechanical orientation treatment, it is suitable for use as a magnetic material for coating film type perpendicular magnetic recording media.
The magnetic materials for the coating film type perpendicular magnetic media must satisfy the following requirements in addition to the fact that they are of a hexagonal system and have uniaxial magnetocrystalline anisotropy.
Namely, in order to conduct recording, reproducing and erasing to magnetic recording media such as magnetic tapes and floppy disks by a magnetic head, the magnetic powder generally must have a moderate coercive force, Hc, of usually about 200 to 2,000 Oe and the highest possible saturation magnetization, .tau.s, of at least 40 emu/g. In addition, its average particle diameter must be at most 0.3 .mu.m in view of recording wavelength and at least 0.01 .mu.m in view of super paramagnetism. Within this range, at most 0 1 .mu.m is preferred in view of noises. Furthermore, it is important to permit provision of high power when it is used as a magnetic recording medium. Therefore, it is preferable for the magnetic powder to have good dispersibility. The dispersibility of a magnetic powder in a magnetic recording medium depends on the shape and surface quality of the magnetic powder, the interaction between the magnetic powder and a resin binder used, etc. It is hence impossible to determine by its shape only whether its dispersibility is good or poor. For this reason, the dispersibility is generally evaluated by mixing the magnetic powder with the resin binder to produce a magnetic recording medium and measuring the glossiness of the surface of the magnetic recording medium. It is therefore desirable that the magnetic powder has high glossiness as a magnetic recording medium.
However, hexagonal barium ferrite (BaFe.sub.12 O.sub.19) magnetic powder known generally has a coercive force of about 5,000 Oe which is too large for a magnetic material for magnetic recording media. Hence, it has been proposed heretofore to reduce the coercive force by substituting a substituting element such as cobalt (Co) for part of the constituent elements of the barium ferrite (for example, U.S. Pat. Nos. 4,341,648 and 4,585,568). According to these methods, the coercive force of the hexagonal barium ferrite magnetic powder can be reduced to a coercive force suitable for use as magnetic recording media by controlling the amount of Co substituted (for example, Japanese Patent Application Laid-Open No. 86103/1980).
The Co-containing hexagonal barium ferrite magnetic powders whose coercive forces have been controlled by the substitution of Co are however accompanied by a problem that when they are used for magnetic recording media, their coercive forces vary to a great extent. For example, the coercive forces of the Co-substituted hexagonal barium ferrite magnetic powders where they have been used as magnetic recording media increase to about at least 1.1 times, and sometimes, about 2.0 times the coercive forces of their corresponding raw magnetic powders used, as shown in Table 1.
TABLE 1 __________________________________________________________________________ Co-containing hexagonal Coercive force Coercive force of Hc of Tape/ barium ferrite magnetic of magnetic magnetic recording Hc of magnetic JPO powder powder, Hc (Oe) medium, Hc (Oe) powder __________________________________________________________________________ 157718/85 Ba.sub.1.0 Fe.sub.8.0 Co.sub.0.40 1110 1210 1.09 168532/86 Ba.sub.1.0 Fe.sub.10.4 Co.sub.0.80 Ti.sub.0.80 845 925 1.09 207720/87 Ba.sub.1.0 Fe.sub.5.73 Co.sub.0.42 Ti.sub.0.42 Si.sub.0.1 515 652 1.27 216922/87 Ba.sub.1.0 Fe.sub.10.2 Co.sub.0.9 Ti.sub.0.45 Sn.sub.0.45 550 800 1.45 235220/87 Ba.sub.1.0 Fe.sub.6.93 Co.sub.0.53 Ti.sub.0.53 1310 1450 1.11 64626/88 Co-substituted barium ferrite 550 620 1.13 __________________________________________________________________________ Note: JPO stands for Japanese Patent Application LaidOpen No.
As shown in Table 1, the coercive forces of the hexagonal barium ferrite magnetic powders containing the elements substituted generally vary when they are formed into magnetic recording media. However, the variation tolerance in coercive force of a magnetic recording medium to be produced must be within (the preset value .+-.20) Oe, preferably, (the preset value .+-.10) Oe from the requirements in performance such as output and noises. For this reason, in order to produce a magnetic recording medium having a coercive force of a desired preset value with high precision within (the preset value .+-.20) Oe, preferably, (the preset value .+-.10) Oe, it is necessary to make an accurate forecast of a relation between the coercive force of a magnetic powder used and the coercive force of the magnetic recording medium produced therefrom, and to produce and use a magnetic powder having the same coercive force as the forecast value.
This will be described specifically by the magnetic powder in Japanese Patent Application Laid-Open No. 207720/1987, which has been shown in Table 1, as an example. In order to produce a magnetic tape having a preset coercive force of 650 Oe with precision of .+-.20 Oe, it is necessary to accurately produce a Co-substituted hexagonal barium ferrite magnetic powder having a coercive force of 512 .+-.15 Oe in advance.
Although the coercive force of the Co-containing hexagonal barium ferrite magnetic powders can be controlled by changing the content of Co, the coercive force of a resulting magnetic powder considerably varies depending upon changes in content of Co even when the amount changed is extremely small (Nikkei New Material, the April 28, 1986 issue, p. 52). Also, its coercive force considerably varies depending upon slight changes in conditions of the production process. It is hence very difficult to produce stably a magnetic powder having a desired coercive force within the limited tolerance as described above.
Furthermore, even if a magnetic powder having a desired coercive force can be produced, it is necessary to control the process so as to prevent the coercive force from varying to a great extent upon the production of a magnetic recording medium from the magnetic powder. However, in the case of the Co-containing hexagonal barium ferrite magnetic powder, its coercive force tends to vary greatly upon the production of the magnetic recording medium depending upon the water content, surface ion density and degree of dispersion of the magnetic powder, or the changes of conditions in the production process of the magnetic recording medium. It is hence extremely difficult to produce a magnetic recording medium having the same coercive force as a preset value with high precision.
On the other hand, various propositions have been made with respect to Co-free barium ferrite magnetic powder for high-density magnetic recording media.
For example, Sn-containing barium ferrite magnetic powder making use of tin (Sn) as a substituent element have been known. These magnetic powders are small in the temperature dependence of coercive force. However, the known Co-free and Sn-containing barium ferrite magnetic powders involve defects that since they generally have an average particle diameter as great as at least 0.085 .mu.m, it is insufficient to make a resulting magnetic recording medium high density, that since their ratio of the maximum diameter to the maximum thickness (hereinafter refer to as the "plate ratio") is as great as at least 10, no high charging rate is attained in a coating medium [for example, Toshiba Review, 40 (13), (1985)], that they are wide in particle size distribution, that since it is difficult to reduce their coercive forces to 1,000 Oe or lower, they cannot be used as magnetic powders for magnetic recording media for which a low coercive force (200-900 Oe) is required, that their saturation magnetization is relatively low, and moreover that since they are poor in dispersibility, no high power can be provided when used as magnetic recording media.
For example, the Co-free and Sn-containing barium ferrite magnetic powders proposed in Japanese Patent Application Laid-Open Nos. 122726/1985 and 174118/1986 are accompanied by defects that the average particle diameter is at least 0.2 .mu.m, the plate ratio is as high as at least 10 and the saturation magnetization is low. With respect to the Co-free and Sn-containing barium ferrite magnetic powder proposed in Japanese Patent Application Laid-Open No. 219720/1986, its coercive force can be reduced to an extent as low as 1,000 Oe or lower, but involves defects that both average particle diameter and plate ratio are as great as at least 0.1 .mu.m and at least 10 respectively.
Although the Co-free and Sn-containing barium ferrite magnetic powder proposed in Japanese Patent Application Laid-Open No. 295236/1986 has merits that the average particle diameter is small and the coercive force is also low, it involves defects that the plate ratio is great and the saturation magnetization is as low as at most 52 emu/g.
The Co-free and Sn-containing barium ferrite magnetic powders proposed in Japanese Patent Application Laid-Open Nos. 193504/1988, 193506/1988 and 193507/1988 are all small in average particle diameter, but involve defects that they are wide in particle size distribution, and since it is difficult to reduce their coercive forces to 1,000 Oe or lower, they cannot be used as magnetic powders for magnetic recording media for which a coercive force as low as 200-900 Oe is required.
In addition, although the Co-free and Sn-containing barium ferrite magnetic powder proposed in Japanese Patent Application Laid-Open No. 310729/1988 has merits that the average particle diameter is small, the saturation magnetization is high and the coercive force is also low, both average particle diameter and plate ratio are great and the dispersibility is poor. It is not preferred to use it as a magnetic powder for magnetic recording media.
As has been described above, the barium ferrite magnetic powders, which are free of Co and contain Sn, are insufficient in performance as the magnetic powders for high-density magnetic recording media.